Synchronous motors are known as electrical drives, such as for dental handpieces, i.e., synchronous machines operated with a motor, in which a rotor in the stator runs in sync with a moved rotary field.
To control such a synchronous machine, it is further known to determine the position of the rotor relative to the rotating stator field during operation. It is known, for example, from DE 195 27 982 AI to provide sensors for this purpose.
A disadvantage is that such a sensor constitutes an additional expense, and that integrating a sensor—for example, in a dental handpiece—can be problematic, given the hygienic requirements.
Another option for determining the rotor position is to measure the electromotive force (EMF) induced by the rotor. For this purpose, the motor windings must be de-energized, however, which takes a certain amount of time, in which drive output is not available. To reduce this time, it is, for example, known to demagnetize, which, however, results in braking torque.
As a result, there is a continual alternation between drive torque and a lack of torque, or even braking torque. These continuous changes in torque can lead to undesirable vibrations and noises.
A method and a device for determining the angular position of the rotor by measuring EMF are described in EP 0 993 108 A2. The method is based upon determining the electromotive force (EMF) induced in a stator winding by the rotor, wherein the other stator windings are short-circuited, and a power shortage close to a suspected zero crossing is generated.
The disadvantage is the dependency of the measurement on the zero crossing of the stator current. If, however, measurement does not occur close to the current zero crossing, this would yield a much longer wait time with regard to demagnetization, depending upon the momentary current value in the winding.
The object of the present invention is to provide a circuit and a method that make it possible to determine the EMF at any time of operating a synchronous machine, and to demagnetize the motor windings as quickly as possible and keep the overall measuring time short.